1. Field of the Invention
The present invention relates to surface treatment method for improving the corrosion resistance of ferrous sintered structural parts employed as various structural parts such as those for automobiles, domestic electric devices, business machines and fishing tackle. More particularly, the invention relates to a surface treatment method comprising a step of applying a compound containing hexavalent chromium to the surfaces of the ferrous sintered parts.
2. Description of the Prior Art
The so-called ferrous sintered structural parts obtained by pressing of sintering material mainly composed of iron powder, are widely used to form, e.g., automobile parts. Such sintered structural parts can be obtained at a low cost with a high accuracy, and in case where a rust preventing ability is required, the sintered parts are subjected to a surface treatment similarly to ingot members.
For example, an extremely simple surface treatment process is plating performed on general ingot members. However, the sintered parts have residual pores, and hence the plating process cannot be performed directly on the surfaces of sintered parts. Namely, the parts must be impregnated by resin prior to plating, to seal the residual pores, in order to properly perform the plating process. Thus, the cost for the surface treatment is increased, and the surfaces of the sintered parts are partially formed by resin, whereby plated layers thus formed are easily removed, due to an insufficient adhesion strength.
Further performed is the so-called steam treatment which is a high-temperature treatment (under 400.degree. to 600.degree. C.) on ferrous sintered bodies in an atmosphere containing heated steam thereby to form dense oxide films mainly composed of Fe.sub.3 O.sub.4 on the surfaces of the sintered parts. This steam treatment is performed at a low cost and the residual pores can be effectively sealed, however this method is not so superior for rust prevention.
Any of the conventional surface treatment methods for sintered parts as hereinabove described cannot be applied in the case where the sintered parts may be in contact with salt water or subjected to salt damage.
On the other hand, general ferrous ingots have no residual pore and hence no serious problem is caused by surface treatment, and the ferrous ingots are subjected to various rust prevention treatment processes. In general, the ferrous ingots are subjected to electric plating utilizing, e.g., zinc, and such electric plating causes environmental pollution.
In place of the electric plating process, U.S. Pat. Nos. 3,708,350 and 3,718,509 disclose methods of applying liquid mediums containing a substance for supplying hexavalent chromium to the surfaces of metal parts to be treated and heating the same for forming corrosion-resistant coating layers on the same surfaces. The coating layers are considered to have such structure that the originally contained hexavalent chromium is reduced to trivalent chromium so that polymers of the trivalent chromium cover the surfaces of, e.g., metal particles of zinc. The surface treatment method employing hexavalent chromium is far superior for rust prevention to the conventional zinc plating method. Further, this surface treatment method is excellent in that it requires no washing step and hence no environmental pollution is caused.
In the aforementioned surface treatment method employing hexavalent chromium experimentally carried out on a ferrous sintered part, however, the coating layer formed as the result rusted in several ten hours due to a salt spray test, and the sintered part was of no practical use. It is believed that this rusting is due to the fact that the ferrous sintered part included continuous and independent residual pores as hereinabove described, and hence air in the sintered metal was expanded by the heat treatment performed after application of a treating solution containing hexavalent chromium to expand the pores in the coating layer formed on the surface, whereby a large number of continuous pores were able to communicate with the surface of the sintered body and the exterior thereof.